The human eye possesses natural accommodation capability. This capability involves relaxation and constriction of the circular ciliary muscle of the eye and related zonules controlled by the ciliary muscle. This relaxation and constriction, controlled by the brain, provides the eye with near and distant vision. This ciliary muscle action is automatic, and varies the shape of the natural crystalline lens to an appropriate optical configuration for focusing light rays entering the eye on the retina. Ciliary muscle relaxation, which is the normal state of the muscle, shapes the human crystalline lens for distant vision. Ciliary muscle contraction deforms the lens for near vision, and the extent of contraction changes the focal length of the lens to the desired endpoint. This change in effective focal length in order to focus on nearby objects is known as accommodation.
Normally the human eye loses its ability to accommodate as individuals reach the age of 40. This condition, known as presbyopia, is typically due to a progressive loss in the elasticity of the lens of the eye, such that the ciliary muscle can no longer exert the necessary deformation of the lens' shape. In addition, the human eye is susceptible to disorders and diseases which attack the crystalline lens, such as cataracts. Cataracts causing partial or complete blindness are typically treated by removing the crystalline lens and replacing it with an intra-ocular lens (IOL).
The conventional solution to the problem of presbyopia is a prescription for reading glasses or, for individuals who already require glasses to correct other refractive errors such as myopia or astigmatism, a prescription of bifocal or multifocal glasses. These fixed variance glasses are provided for the subject's eye to have better vision of objects at different distances.